Method and structure for connecting a floating structure with rope anchor lines to the seabed

ABSTRACT

A method of connecting a first floating structure to the seabed, includes the steps of: providing a second floating structure, anchored to first and second anchoring points respectively on the sea bed via at least two anchor lines, attaching the second floating structure to the first structure via a pulling device, on the side of the first anchor line, displacing the first floating structure away from the second anchoring point towards the first anchoring point, disconnecting the first anchor line from the second floating structure while maintaining a pulling force on the second anchor line via the pulling device, and attaching the first anchor line to the first floating structure. By interconnecting a temporary buoy to the vessel to be anchored via a tension member, the anchor lines of the buoy and the vessel can remain under tension at all times whereby damage to the anchor lines is prevented.

BACKGROUND OF THE INVENTION

The invention relates to a method and structure for anchoring a floatingstructure, such as a hydrocarbon storage and/or processing vessel, atanker, barge, SPAR buoy or a mooring buoy with anchor lines containinglarge rope sections to the sea bed.

Upon installation of an offshore project, floating production storageand offloading vessels (FPSO), floating production, drilling, storageand offloading vessels (FPDSO), floating production, workover, storageand offloading vessels (FPWSO), Spar buoys, catenary anchor leg mooring(CALM) buoys, semi-submersibles or other hydrocarbon storage and/orprocessing vessels are moored to the sea bed via anchor lines, and areconnected to a subsea oil or gas well via one or more product risers.Especially in water depths of over 300 m, rope anchor lines are used,such as steel wire rope or polyester rope. If these anchor lines becomeslack during installation, they can bend and kink, such that the anchorlines are weakened, or may even break. Placing the rope anchor lines onthe seabed during installation is not preferred in view of possibledamage to the anchor lines.

Upon installation, a critical time path can be followed in which theanchor lines, product risers and hydrocarbon production and/or storagevessel need all be installed and hooked up simultaneously prior toproduction. Alternatively, first installing the anchor lines, followedby hook up of the vessel to the anchor lines and product risers, resultsin the problem of temporary abandonment and recovery of the anchor linesand consequent damage to the anchor lines.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodof connecting a floating structure to the seabed with anchor timescontaining tensioned rope sections and preventing damage to the anchorlines.

It is a further object to provide a method of connecting a floatingstructure to the seabed with anchor lines containing large ropesections, while the anchor lines remain in a tensioned state.

It is in particular an object of the present invention to provide aninstallation method and structure for a hydrocarbon production and/orstorage vessel, allowing connection of mooring legs and risers to avessel in one continuous operation.

It is again an object of the present invention to provide a method andstructure by which different floating structures can be connected to theseabed, and/or can mutually be connected in an efficient manner.

It is an another object of the present invention to provide a method ofinstalling a vessel in deep water, such as in water depths of 300 m andmore, using rope anchor lines.

Thereto the method according to the present invention comprises thesteps of:

providing a second floating structure, anchored to first and secondanchoring points respectively on the sea bed via at least two anchorlines, each anchor line comprising a large tensioned rope section,

-   -   attaching the second floating structure to the first structure        via a member on the side of the first anchor line,    -   disconnecting the first anchor line from the second floating        structure and attaching the first anchor line to the first        floating structure, while maintaining the positions of the first        and second floating structure relative to each other via the        member.

The term “floating structure” as is used herein is intended to comprise,barges, FSO's, FPSO's, FPDSO's, FPWSO's, temporary buoys, CALM buoys,Spar buoys and mooring buoys, semi-submersibles, shuttle tankers, etc.

The second floating structure may for instance be a temporary buoy orbarge which is anchored to the seabed. One or more product risers may beconnected to the temporary buoy.

By interconnecting the temporary buoy and the vessel to be anchored viaa tension member such as a hawser, the anchor lines on the buoy andvessel can remain under tension after they are transferred from the buoyto the vessel. Hereby damage to the anchor lines is prevented, withoutthe need for temporarily abandoning the anchor lines on the seabed. Themethod allows the use of synthetic ropes like polyester ropes and istherefore especially suitable for deep water. When the anchor lines andproduct risers have been transferred from the temporary buoy to thevessel, the temporary buoy can be removed from the installation site.

By using a temporary installation buoy or barge, simultaneous hook upand installation of the vessel, such as an FPSO can be carried out uponarrival of the FPSO on the site, independently of the constructionschedule of the FPSO. Hereby the time path of fabrication andinstallation of the risers and the FPSO is made more flexible and lesscritical in terms of first oil and contingencies, and the installationcosts can be reduced.

By the method according to the present invention, the pre-installationof the anchor lines to a temporary buoy or barge, allows removing ofmore than 90% of the permanent stretch from the anchor lines before theyare transfer to and hooked-up on a permanent floating structure. Thiscan be done by tensioning cycling of the anchor lines duringinstallation, leaving the anchor lines on the temporary buoysufficiently long time and by re-tensioning the anchor lines on saidbuoy to remove the permanent stretch. To this end the anchor lines onthe temporary buoy can be equipped with a chain part for progressivelypulling in the anchor lines on the temporary buoy, which chain parts maylater be re-used. The second vessel can after removal of the stretchfrom the anchor lines be attached to these anchor lines without anychain parts to be pulled in to compensate for length variations, wherebythe need for tensioning operations and chain adjustment equipment oreven a heavy chain table on the second vessel, is obviated.

The floating structure to be anchored to the sea bed may be a mooringbuoy for mooring to for instance an FPSO, wherein the anchor lines ofthe temporary buoy are such as to be adapted to anchoring the mooringbuoy and the vessel moored thereto.

The method of the present invention may also be used for change out ofan already moored buoy or vessel. The anchor lines and product risers ofsuch a vessel, such as a turret or spread moored vessel, may beconnected to a temporary buoy or barge, for exchange or maintenance ofthe vessel.

Also, a spread moored or turret moored vessel may be connected to amooring buoy for shuttle tankers via a horizontal duct, one end of whichis attached to a temporary buoy. The temporary buoy also carries one ormore product risers and anchor lines, which are all transferred to thespread moored or turret moored vessel by the method of the presentinvention.

Finally, the method may be used to connect a shuttle tanker to apermanently moored FPSO for tandem offloading or for side by sideoffloading.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the method according to the present invention willby way of example be explained in detail with reference to theaccompanying drawings. In the drawings:

FIGS. 1 a-1 f show the different stages of anchoring a permanent mooringbuoy using a temporary buoy or barge according to the present invention,FIG. 2 shows a method of change out of a vessel by a temporary buoy,FIG. 3 shows an installation method of a mid depth transfer ductaccording to the present invention, FIGS. 4 a and 4 b show aninstallation method of a shuttle tanker for tandem offloading, FIG. 5 ashow a side view of a barge for temporary mooring purposes, and FIG. 5 bshows a top view of the barge of FIG. 5 a.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an installation method for installing a vessel such as amooring buoy 1 or an FPSO. In FIG. 1 a is a temporary buoy 2 moored tothe seabed 3 via polyester anchor lines 4,5 to anchoring points 6,7. Theanchor lines 4,5 may be between 300 and 5000 m in length. The buoy 2will be of the dimensions suitable of supporting the suspended weightsof the mooring lines, risers and other equipment i.e. winches, andpower-packs. A product riser 11 is connected on one end to a sub-seahydrocarbon well, and is with its free end connected to the buoy 2. Atug 9 pulls the mooring buoy 1 by a hawser 10 to a position close to thestock buoy 2. Next, the mooring buoy 1 is connected to the stock buoy 2via a member which can take up tension forces, for example a hawser 13.The hawser 13 could have a length of at least 20 meter. For redundancyor stability reasons it could be necessary to have more than one hawserbetween the floating bodies.

As is shown in FIG. 1 b, the tug avoid collision of the buoys and keepbuoy 1 away from the second anchor point 7 by pulling in the directionof the arrow T. Next, the anchor line 4 is detached from the buoy 2 andre-attached to the mooring buoy 1. The transfer of the anchor line 4 canbe done via a cable 12 connected to a winch on buoy 1 and which isconnected to the anchor line 4 via an connection point like a tri-plateor connected directly to a shackle of an upper chain section of theanchor line 4. The anchor line 4 is connected to a winch on buoy 2 andthe anchor line 4 is released while at the same moment line 12 is pulledin on buoy 1, which will transfer the anchor line 4 to the buoy 1.Alternatively the cable 12 can be pulled on board of the buoy 1 tug 9which keeps the buoy 1 in place and afterwards the anchor line 4 can betransferred to the buoy 1. The anchor line 4 and/or the cable 12 canalso be pulled in by a tug not connected to any one of the buoys, andtransfer the anchor line 4 to buoy 1.

During the operation of transferring the first anchor line or lines, thetug 9 can exert a pulling force on the buoy 2 in the direction of thearrow T to maintain the positions of the buoys relative to each otherand to the seabed and avoiding the anchor line 5 to get to slack, kinkor touch the seabed (see in FIGS. 1 b and 1 c).

After the first anchor line or lines are transferred, more anchor linescan be transferred without a tug pulling the buoy 1 as the whole mooringsystem will be basically a stabilized system due to the tension member13 between the buoys, as is shown in FIG. 1 c. It is preferred toconnect the anchor line 4 or the first anchor lines to the mooring buoy1 in the part that is past the centerline away from the buoy 2. The restof the anchor lines can now be transferred from buoy 2 to buoy 1 exceptfor a last anchor line 5 which will keep the mooring configurationstable.

The risers 11 will be transferred from temporary buoy 2 to buoy 1 in thesame manner as the anchor lines. This transfer can be performed beforethe last anchor line when is transferred and preferably when there a forexample an equal number of anchor lines connected to each buoy (when themooring configuration of the interconnected buoys 1,2 is the moststable).

Before the last anchor line or lines are transferred, the hawser 10 isdetached from the mooring buoy 1, and is attached to the temporary orstock buoy 2, on the opposite side of the transferred anchor line 47 asis shown in FIG. 1 d. The tug maintains a putting force, such as forinstance 1 ton on the temporary buoy 2, such that the anchor line 5 canbe disconnected and transferred to the mooring buoy 1, while keepingboth buoys in position, as shown in FIG. 1 e. After all the anchor lineare transferred using one of the earlier mentioned methods, the hawser13 is disconnected and the temporary buoy 2 is removed from the site bythe tug 9.

During the installation or anchor line transfer procedure, the anchorfines 4,5 are maintained at such a tension that they do not become toslack so that they can touch the seabed and the tension does not fallbelow the suppliers recommended value, e.g. 15% of the minimum breakingload.

FIG. 1 f shows a situation which is preferable for small size buoychange out. The permanent mooring buoy 1 is moored via fenders side byside against the temporary buoy 2 with at least one tension member 13,which connect the buoys. In this configuration there is no need to pullon one of the buoys with a tug boat to avoid collision and/or tomaintain the relative positions of both buoys as the relative positionsof the buoys is secured by the tension member or hawser 13. The anchorlines 4 and 5 can be transferred directly from the temporary buoy 2 tothe permanent buoy 1 (or reverse).

The same procedure as described with respect to FIGS. 1 a-1 f can beused to chance out a permanent mooring buoy 2, with a temporary or a newmooring buoy 1, for maintenance or repair purposes.

FIG. 2 shows a spread moored or turret moored vessel 18, which isconnected to the sea bed via anchor lines 19,20 to anchor points 22, 23.A number of product risers 21 are connected to the vessel 18. The vesselis disconnected from the risers 21 and the anchor lines 19,20 in thesame way as described for FIGS. 1 a-1 e, the risers and anchor linesbeing connected to a temporary buoy or barge 16, which is attached totug 15 via hawser 17.

FIG. 3 shows a method of installation of a mid water pipe, of the typeas described in WO99/62762 in the name of the applicant. A mooring buoy27 for offloading to a shuttle tanker is anchored to the seabed atanchor points 40,41 via anchor lines 35,39, and carries the end of asubstantially horizontal transfer duct 28. The other end of the duct 28is carried by temporary buoy or barge 26. The buoy 26 is attached to theseabed via anchor lines 30,31, extending to anchor points 32, 33. One ormore product risers 29 are attached to the buoy 26. A vessel 25 to bespread or turret moored is connected to the buoy 26 via hawser 35. Thevessel 25 is attached to tug 37 via hawser 36. The vessel 25 is anchoredto anchor lines 30,31 and is connected to riser 29 in the same way asdescribed in FIGS. 1 a-1 e. The buoy 27 may be installed in the sameway.

In FIG. 4 a a shuttle tanker 41 is moored to the stern of vessel 40 viaa hawser 46. Tug 42 pulls the tanker 41 in the direction indicated bythe arrow T, so that the relative positions of both vessels will bemaintained. The anchor line 45 is thereafter detached from the vessel40, such as shown in FIG. 4 b, and is transferred to the shuttle tanker41 to obtain the tandem offloading configuration of FIG. 4 b.

Basically the same procedure can be used for a side by side mooringconfiguration of two vessels where at least two relative long hawsersfunction as tension members between the vessels (not shown). In thiscase one or more tugboats are pulling one vessel sideward away from themoored vessel and thereafter one or more stern and one bow anchor linesare transferred from the moored vessel to the other vessel.

Finally, FIGS. 5 a and 5 b show a special designed barge 50, 50′ fortemporary mooring purposes. The barge comprises connection points 54,54′ for one or more mooring lines at the corners of the barge. There areone or more connection points 57 for connecting hawsers to the barge.The barge has a deck storage capacity for reels 51, 51′ with mooringropes, a reel motor, a reel drive system and multiple anchors 52, 52′which for example could be suction anchors held in a skidding system.The barge could also have lifting means 53, 53′, 55 to lift the anchorfrom the deck and lower the anchor with the rope from the mooring ropereel to the seabed, but this lifting and lowering could also be done bya separate installation vessel. The barge comprises also a winch, forexample a 200 tons winch, for lowering, (cycle-) tensioning andre-tensioning the installed mooring lines connected to the barge 50,50′. The (cycle-) tensioning and re-tension can take place duringinstallation of the moorings lines and/or in time when all the mooringlines are installed. This will remove all or most of the permanentstretch from the mooring lines before the mooring lines are transferredand hooked-up to a permanent floating structure. The winch can tensionand/or re-tension each anchor line apart or groups of anchor via a decksheave construction. A barge with this equipment and a deck space for 20suction anchors and 8 reels for mooring lines could be for example 90meters long and 25 meters width. The barge could have any shape andcould for example also be a special equipped installation vessel.

1. A method of connecting a first floating structure (1, 16, 25, 41) tothe sea bed, comprising the steps of: providing a first floatingstructure floating on the sea; providing a second floating structure(2,18, 26, 40), anchored to first and second anchoring points (6, 7; 22,23; 32,33;47) respectively on the sea bed via at least two anchor lines(4,5; 19, 20; 30,31;43,45), each anchor line comprising a largetensioned rope section and being with a top part connected to the secondfloating structure; attaching the second floating structure to the firstfloating structure via a member (13, 24, 35, 46), on the side of thefirst anchor line; and disconnecting the top part of the first anchorline (4,20,31,45) from the second floating structure (2,18, 26, 40) andattaching the top part of the first anchor line (4,20,31,45) to thefirst floating structure_(1, 16, 25, 41) while maintaining the positionof the first and second floating structures relative to each other byexerting a pulling force on the second anchor line (5, 19, 30, 43) ofthe second floating structure.
 2. The method according to claim 1,comprising the step of disconnecting the second anchor line (5,19,30,43)from the second floating structure (2,18,26, 40) and connecting it tothe first floating structure (1,16,25, 41), while maintaining theposition of the first and second floating structure relative to eachother via the member (13,24, 35,46).
 3. The method according to claim 1,wherein the first anchor line (4,20,31,45) is attached to the firstfloating structure (1,16,25,41) in the part past the centerline awayfrom the second floating structure (2,18,26,40) so that the member(13,24,35,46) will be tensioned.
 4. The method according to claim 1,wherein the second floating structure comprises at least one productriser (11,21,29) extending between a sub sea hydrocarbon structure andthe second floating structure (2,18,26,40), which product riser isdecoupled from the second floating structure (2,18,26,40) and is coupledto the first floating structure (1,16,25,41).
 5. The method according toclaim 4, wherein the anchor lines are left to stretch and arere-tensioned on the second floating structure (2,18,26,40) at least onetime, before transferring the anchor lines to the first floatingstructure (1,16,25,41).
 6. The method according to claim 1, wherein theanchor lines comprise polyester rope.
 7. The method according to claim1, the first floating structure comprising a mooring buoy (1,16) or ahydrocarbon storage and/or processing vessel (25), the second floatingstructure (2,18,26) being removed after transfer of the anchor lines tothe first floating structure (1,16,25).
 8. The method according to claim1, wherein the first and second floating structures (1,2) each comprisea mooring buoy.
 9. The method according to claim 1, wherein the secondfloating structure (18) comprises a mooring buoy or a hydrocarbonstorage and/or processing vessel, the second floating structure beingremoved after transfer of the anchor lines (19,20) and product riser(21) to the first floating structure (16).
 10. The method according toclaim 1, wherein the second floating structure comprises a first andsecond buoy (26,27), each buoy connected to the sea bed via anchor lines(30,31;38,39) and mutually connected by a submerged transfer duct (28),the first buoy (27) being adapted for mooring to a tanker (25), thesecond buoy (26) comprising at least one product riser (29) connected toa sub sea hydrocarbon structure, wherein the anchor lines (30,31) andthe at least one product riser (29) of the second buoy are transferredto the first floating structure (25).
 11. The method according to claim1, wherein the second floating structure comprises a first and secondfloating member (26,27), each floating member connected to the sea bedvia anchor lines (30,31;38,39) and mutually connected by a submergedtransfer duct₁₃ (28), the second member (26) comprising at least oneproduct riser (27) connected to subsea hydrocarbon structure, whereinthe anchor lines (30,31) and the at least one product riser (29) of thefirst and second floating members (26,27) are transferred to the firstfloating structure (25), and to a third floating structure respectively.12. The method according to claim 1, the second floating structurecomprising a hydrocarbon storage and/or processing vessel (40), thefirst floating structure (41) comprising a tanker, wherein the tensionmember (46) remains attached to both floating structures (40,41) aftertransfer of the at least one anchor line (45) from the second floatingstructure (40) to the first floating structure (41).
 13. The methodaccording to claim 1, wherein the relative position of the firstfloating structure (1,16,25,41) and second floating structure(2,18,26,40) during transfer of at least the first mooring line(4,20,31,45) and the second or last mooring line (5,19,30,43) is ensuredby a tug boat (9,15,37,42) pulling at one of the floating structures viaa hawser (10,17,36).
 14. The method according to claim 1, wherein thetension member (13,24,35,46) is a hawser of at least 20 meters long. 15.The method according to claim 1, each anchor line comprising a largetensioned synthetic rope section, the method further comprising thesteps of: stretching and re-tensioning the anchor lines of the secondfloating structure (2,18,26,40) at least one time; and after at least50% of the permanent stretch of the synthetic rope section is taken out,disconnecting the first anchor line (4,20,31,45) to the first floatingstructure (1,16,25,41).